This invention relates to the recovery of ammonia from underground storage caverns, for example caverns made in natural salt deposits.
Because of the seasonal demand for ammonia and ammonia products, for example in the fertilizer industry, manufacturing capacity during off-seasons exceeds the rate of consumption. It is economic to produce excess ammonia at these times and store this excess for use during times of peak demand. Although underground storage of ammonia is known, this method of storage has not generally gained favour over use of above ground storage tanks, some of which are large enough to hold 40,000 tons of ammonia. Although many precautions are taken to provide safe above ground storage of ammonia, such as storage in remote areas on stable ground, there is a hazard of accidental release of large quantities of ammonia to the atmosphere. Underground storage reduces this hazard substantially. However, known processes for the recovery of ammonia from underground caverns have disadvantages in their need of elaborate heat exchange and gas purification means. Some of the problems arise because the underground caverns are made from natural salt deposits and the salt contaminates the ammonia, particularly when it is withdrawn from the cavern in liquid form.
In U.S. Pat. No. 2,732,334 (Pollock), gaseous ammonia is withdrawn from the region above the surface of liquefied ammonia stored in an underground salt cavern. The withdrawn gaseous ammonia is compressed without liquefying, and is returned to the cavern where it is passed through a heat exchange conduit immersed in the liquefied ammonia. The gaseous ammonia condenses in the immersed conduit, giving up heat which evaporates some of the stored liquid. The condensed ammonia is then pumped by a submerged pump from the immersed conduit to an above ground outlet product line. This process therefore requires the presence of a heat exchange conduit and a pump submerged in the liquefied ammonia in the cavern.
In one embodiment of the process described in U.S. Pat. No. 2,713,775 (Cottle), an inert liquid, which is denser than ammonia and has a higher boiling point, such as pentane, is added to a salt cavern containing liquefied ammonia to cover heat exchanger piping and a pump. Ammonia gas is removed from above the liquid ammonia, compressed and passed through the submerged piping. Heat given up to the pentane by the ammonia gas is, in turn, given up to the layer of liquid ammonia above it, some of which is thereby evaporated. The ammonia gas in the submerged piping condenses as it loses heat to the pentane and is pumped to the surface, where it is separated from any entrained pentane liquid. In another embodiment, liquid ammonia is removed from the cavern and passed through an evaporator in which a pentane layer is used to separate dissolved salt. The complications involved by the use of an inert liquid such as pentane are readily apparent.
In the process described in U.S. Pat. No. 2,878,165 (Cottle), ammonia gas is pumped into a salt cavern to move salt-containing liquid ammonia into an above ground purification system where salt is separated. U.S. Pat. No. 2,901,403 (Adams) provides a process in which an inert gas, such as off-gas from ammonia synthesis, is introduced into a salt cavern containing liquid ammonia and dissolved salt with sufficient force to lift the liquid into an above ground separator, where the inert gas is separated and recycled. Ammonia is then distilled from the salt solution. Again, the complicated nature of these processes is readily apparent.
It is therefore an object of the invention to provide a process for recovering ammonia from an underground cavern, such as a salt cavern, in a relatively uncontaminated state and with a less complicated withdrawal system than those previously known.